I have a book that is kind of old that mentions a disease very similar to what your frog was suffering from. The disease is called Dropsy. It describes symtoms as a abnormal accumulation of serous bodily fluids. Possibly caused by bacteria/but more so improper body metabolism. Caused by inadequate diet or poor climate maintenance. Bloating or soft dermal abnormalities. Usually around the abdominal region, but can form in other sensitive areas like near the eyes. Vet is best. Treatment suggests to sterilize a needle and puncture the spot to drain the fluid. Definetly go to a vet. Sounds to risky. That is if it reoccurs and doesn't get better.